Number sense, or the ability to quickly ascertain and compare quantities, is correlated with general mathematical ability, and very poor number sense is the hallmark of dyscalculia. One online measure of number sense, the Panamath test (developed by Johns Hopkins University psychologist Justin Halberda and his student Ryan Ly), is freely available at www.panamath.org. For a taste of the test, glance at the first box below and quickly decide whether it has more blue or yellow dots. Then do the same for the other box--a more challenging task.